Micro electromechanical systems (MEMS), and specifically, microoptical electromechanical systems (MOEMS), are typically fabricated at the wafer and/or sub-wafer level. Hundreds of devices are typically manufactured on a single 4-inch or 6-inch wafer, for example. Sometimes these wafers are cut into multi-device coupons for further processing before the MOEMS optical devices are finally singulated into the individual device dies.
Because MEMS devices typically include cantilevered features and/or deflectable surfaces, provisions must be made for protecting devices during the singulation process. Singulation is a relatively violent process that typically includes sawing and/or cleaving a precursor structure comprising multiple MEMS devices into individual MEMS device dies. These steps of sawing and cleaving, and possibly etching, produce particulate matter that can interfere with the operation of the MEMS devices.
Various strategies have been proposed for protecting MEMS devices, such as accelerometers during singulation. The strategies typically combine a combination of frontside and/or backside sawing and/or etching. Sometimes water-soluble protective coatings are deposited over the MEMS structure and then later partially removed during a die sawing process. In still other examples, special two-layer dicing tape systems are used that encapsulate the MEMS structure for the die sawing process.
Existing singulation technology assumes that the MEMS device structure is only present on one side of the precursor structure. This is true for common MEMS accelerometers. For MOEMS devices, however, this assumption can sometimes be invalid. Optical ports are sometimes desirable or required through the backside of the MOEMS device to provide for transmission through the device. Existing singulation technology does not address this device configuration, since the particulate can enter the MOEMS device from both sides of the wafer. Moreover, attempts to perform some aspects of the singulation process prior to the release of the membrane, for example, yield new problems associated with electrostatic charge accumulation, during optical coating deposition, on electrically isolated device dies. This can result in membrane snap-on the handle wafer material and its stiction adhesion.
In general, according to one aspect, the present invention is directed to a process for singulating MOEMS optical devices from a precursor structure. This precursor structure comprises device material, in which movable optical structures are formed, and handle material, through which optical ports are formed to provide for optical access to the movable optical structures. The inventive process comprises coating a frontside and a backside of the precursor structure with protection material. The precursor structure is then attached to a substrate, which is typically a dicing tape, and the precursor structure separated into individual optical devices by a process, including die sawing. Thereafter, the optical devices are removed from the tape and the protection material removed from the optical devices.
In the preferred embodiment, the step of coating the frontside and the backside of the precursor structure comprises spraying the protection material on both the frontside and the backside of the structure. Preferably, a photoresist material is used, which is later removed by dissolving the photoresist layer in a bath. In one implementation, the bath is acetone. Also, circulation is preferably employed so that, as particulate matter is freed from the precursor structure due to the dissolving of the protection material, the particulate matter is swept away.
In the preferred embodiment, the precursor structure is separated into the individual optical devices by die sawing completely through the handle material and device material. In other implementations, however, die sawing is performed partially through the handle material and then cleaving through the remaining material to completely singulate the optical devices.
Also, according to the preferred embodiment, the dicing tape is UV (ultra-violet) curable, with the frontside and backside of the precursor structure being coated prior to attaching the precursor structure to the dicing tape. Consequently, the adhesion of the dicing tape is reduced through UV exposure. The singulated devices are then placed into a waffle pack jig for the subsequent removal of the protection material.
In general, according to another aspect, the invention also features a processing jig for singulated MOEMS devices. This jig has the form of a waffle pack comprising an array of cells for receiving singulated optical devices. A top is placed over the waffle pack to retain the MOEMS devices in their respective cells. Fluid ports are provided in each device cell for facilitating fluid circulation through the cells.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.